I. Field of the Invention
The present invention relates to helical antennas. More particularly, the present invention relates to a novel and improved antenna assembly, a novel and improved assembly tool and a related method for making helical antennas having coupled radiator segments.
II. Related Art
Contemporary personal communication devices are enjoying widespread use in numerous mobile and portable applications. With traditional mobile applications, the desire to minimize the size of the communication device, such as a mobile telephone for example, led to a moderate level of downsizing. However, as the portable, hand-held applications increase in popularity, the demand for smaller and smaller devices increases dramatically. Recent developments in processor technology, battery technology and communications technology have enabled the size and weight of the portable device to be reduced drastically over the past several years.
One area which affects the size and weight of the portable communications device is the device's antenna. The size and weight of the antenna play an important role in downsizing the communication device. Size of the device is not the only factor that needs to be considered in designing antennas for portable applications. Another factor to be considered in designing antennas is attenuation and/or blockage effects resulting from the proximity of the user's head to the antenna during normal operations. Yet another factor is the characteristics of the communication link, such as, for example, desired radiation patterns and operating frequencies.
An antenna that finds widespread usage in satellite communication systems is the helical antenna. One reason for the helical antenna's popularity in satellite communication systems is its ability to produce and receive circularly-polarized radiation employed in such systems. Additionally, because the helical antenna is capable of producing a radiation pattern that is nearly hemispherical, the helical antenna is particularly well suited to applications in mobile satellite communication systems and in satellite navigational systems.
Conventional helical antennas are made by twisting the radiators of the antenna into a helical structure. A common helical antenna is the quadrifilar helical antenna which utilizes four radiators spaced equally around a core and excited in phase quadrature (i.e., the radiators are excited by signals that differ in phase by one quarter of a period or 90.degree.). The length of the radiators is typically an integer multiple of a quarter wavelength of the operating frequency of the communication device. The radiation patterns are typically adjusted by varying the pitch of the radiator, the length of the radiator (in integer multiples of a quarter-wavelength), and the diameter of the core.
Conventional helical antennas can be made using wire or strip technology. With strip technology, the radiators of the antenna are etched or deposited onto a thin, flexible substrate. The radiators are positioned such that they are parallel to each other, but at an obtuse angle to the sides (or edges) of the substrate. The substrate is then formed, or rolled, into a cylindrical, conical, or other appropriate shape causing the strip radiators to form a helix. Typically, a plastic cover or radome is placed over the antenna elements to protect them from damage.
This conventional strip-made helical antenna, however, is difficult to manufacture. Among the problems associated with conventional helical antennas is the difficulty of ensuring that the field inside the helix is undistorted and is always axially symmetric. This problem is due to the fact that, in conventional strip-made helical antennas, the center feed, bandpass receive filter and low noise amplifier, are all etched or deposited onto a thin flexible substrate which is an extension of the radiator substrate. This arrangement can lead to cracking and/or breakage of the center feed during handling and assembly.
In addition, a helical antenna is difficult to manufacture in effective yields. Because it is formed on the same flexible substrate as the helical radiator elements, the center feed is movable within the cylinder of the helix. The center feed may end up being closer to one side of the cylinder formed by the radiator helix than to the other. This leads to the undesirable effect of creating an uneven radiation pattern in the antenna. Having the center feed coincident with the axis of the helical antenna minimizes the impact of this member on the radiation patterns of the antenna. A still further problem relates to the radome. Because of the way the antenna elements are formed, the radome may be spaced unevenly from the helically wound radiators. This tends to distort the radiation pattern and lowers the efficiency of the antenna.
What is needed, therefore, is a helical antenna that is easy to manufacture, that can be manufactured with high yields cost effectively, and which eliminates the problems associated with conventional helical antennas. Also what is needed is a tool or assembly technique that simplifies the consistent manufacture of high quality helical antennas. As will be made clear below, these goals are achieved with the present invention.